Validate journal location before export process is started.

Check to make sure give path exists and there is an entries and photos
directory inside.
Fix problem with putting empty tags in entry.

If use keywords as tags option was selected and there were no keywords
for the picture or if apply specific tags was selected and nothing was
in the text box, empty tags would be put into the entry.
Specify what values are necessary in the tags text field.

It seems ambiguous what values are necessary for the text field
when option is selected to apply specific tags to an entry. Added
label to specify that comma separated values are required.